<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button>按钮一</button><button>按钮二</button><button>按钮三</button>
  <script>
    var aBtns = document.getElementsByTagName("button")
    for (var i = 0; i < aBtns.length; i++) {
      aBtns[i].index = i
      aBtns[i].onclick = function() {
        console.log(this.index);
      }
    }
  </script>
</body>

</html>